The invention is described below as it applies to paper-making apparatus, because of its particular value in that context. However, in some respects the invention is applicable to other uses in which material to be heat treated is carried into contact with a heated cylinder.
The Fourdrinier process of paper making involves a succession of phases. Initially a slurry of cellulose fibers in water is distributed on a screen and some of the water is drained off. A web is formed which is then transported by a felt or a succession of felts to pass a number of nip rollers in a press section. The felt and the formed web are squeezed between the nip rollers to extract water mechanically. In current practice, the web leaving the press section contains from 35 to 45% solids. The web then passes through a dryer section consisting of heated cylinders, in which the water content of the web is reduced by evaporation to roughly that of the finished paper.
Size coaters often follow the dryer section, followed by afterdryers and calenders, ending with the reel. The dryers and afterdryer sections may contain 60 or more heated cylinders. A felt is used to hold the paper firmly against many of the heated dryer cylinders, for assuring contact of the web with the heated surface and thereby promoting drying efficiency. Drying the web is the result of evaporation, caused by conduction of heat from the cylinders into the fibrous moisture-laden web. The term moisture-laden refers to water in all forms carried by the web, as free water or as moisture bound to the web""s fibers.
In the U.S., roughly half the production is paperboard, which is formed into substantially thicker and heavier sheets than paper and newsprint. Many paperboard machines do not use papermaker""s felts in the final dryer sections, because they are not necessary.
When the cold web enters the dryer section, fibers may be picked out of the web, adhering to the hot dryer cylinders. To suppress that effect, the temperature of the first series of dryer cylinders is comparatively low. Each successive cylinder""s temperature is progressively higher until the sheet has been warmed up sufficiently for the web to encounter a hot dryer cylinder without concern for xe2x80x9cpickingxe2x80x9d of fibers.
The following series of dryer cylinders effect a constant rate of drying. In this region the cylinders"" temperature may be uniform. The paper making machine includes a falling rate zone that follows after the constant rate zone. The temperature of the steam in the successive cylinders of the falling rate zone is increased to 370xc2x0 F. (187xc2x0 C.). This is the practical upper limit for cylinders heated by steam under pressure. In the falling rate zone, the rate of evaporation declines progressively, due to the relatively dry condition of the web; in that condition, the web is a poor heat conductor, so that the transfer of heat to the web declines.
The highest pressure steam is typically delivered to the final dryer section, and a cascade steam system delivers reduced temperature steam upstream, to each cylinder of the series of dryer cylinders. It is complicated and expensive to provide steam at a pressure such that a specified high temperature is maintained in each of the cylinders. This is especially true when temperature changes are to be made.
Steam-heated cylinders are massive, both because of their large size and substantial wall thickness, They are usually made of gray cast iron for economy, and their walls are quite thick; e.g., 1xe2x80x3 to 2xe2x80x3 (25 mm. to 51 mm.) or more, to withstand the high internal steam pressure. A web may be 25 ft. (7.6 m.) wide, requiring cylinders that are slightly longer. The web may travel at 3300 ft./min. (1000 m./min.), or roughly 37 miles/hr. (60 km./hr.). That speed is impressive. The dryer section typically includes 60 cylinders. By any standard, the capital investment in a paper making machine is huge, and a considerable amount of space is needed.
Various types of paper making apparatus differ from that outlined above. For example, the xe2x80x9cYankeexe2x80x9d type is characterized by inclusion of one very large diameter dryer cylinder; e.g., a diameter of 12 ft. to 18 ft. (3.6 m. to 5.5 m.). There, the wall thickness is particularly great, to withstand the pressure of the contained high-temperature steam and to allow for periodic grinding to restore surface smoothness.
The highest temperature of any steam-heated cylinder is limited by the corresponding pressure of steam that can be safely contained within the cylinder. The maximum internal steam temperature of a dryer cylinder (see above) is approximately 370xc2x0 F. (187xc2x0 C.) because of concern for the high steam pressure. It has been widely recognized that higher regulated temperatures, if feasible, would accelerate the drying process and would reduce substantially the number of dryer cylinders required.
Paper machine drying sections, worldwide, are almost universally heated by steam under pressure. Accordingly, it is appropriate to consider such apparatus in further detail, as a basis for appraising the advance in the art represented by the present invention.
As noted above, the temperature of a drying cylinder in a paper making machine is not determined by that which would be desirable from the point of view of performance, but by the limitations of cylinders heated by steam to withstand high pressures safely. This is evidenced by the large numbers of drying cylinders required in high-speed paper making machines or by the limited machine speed with lower temperature cylinders performing the drying function. Cylinders heated by steam under pressure have other significant limitations.
The external surface of a steam-heated cylinder responds slowly to an adjustment in steam pressure. This slow response time is manifested, for example, by the many minutes needed to bring the paper making machine from a cold start to full-speed operation. It is also manifested by the delayed change of a cylinder""s external temperature in response to an adjustment in steam pressure.
It is virtually impossible to regulate the temperature of a cylinder wall from point-to-point along its length, for developing a desired temperature profile across the width of a web being dried. It is well-known that steam cylinder dryers are hotter at the ends, where no moist paper is present to absorb thermal energy from the cylindrical shell and from the end walls of the cylinder. Complicated, cumbersome arrangements have been proposed in an effort to compensate for the otherwise excessive cylinder temperatures at the margins of the web. These have been intended to control edge curl caused by unrestrained and excessive drying at the edges of the sheet. However, no easy, pitaical way has been found for varying the cross-machine temperature profile of a cylinder heated by steam under pressure.
The cross-machine moisture profile of a web emerging from the main dryer in a machine for producing paper and paperboard tends to develop non-uniformity not only at the margins but also at other portions of its width. This results from cumulative effects in the forming, press, and dryer sections. A web with moisture streaks is poorly suited to being coated as with size; moisture variations of the web cause the coating to be non-uniform. Also, a web whose cross-machine moisture profile is non-uniform has a tendency to render the calendering non-uniform.
The foregoing and other characteristics of a machine for making paper or paperboard, having dryer cylinders heated by steam under pressure, are impaired by some of the traits of the cylinder wall. Transfer of heat from the steam to the outer surface of the cylinder which contacts the web is impeded by many factors, including:
a) The considerable thickness of the cylinder wall needed for containing steam under the high pressure corresponding to the steam temperature, noting that the actual wall thickness is greater by a safety factor of 2.8 times that theoretically required for withstanding the steam pressure;
b) The poor heat conductivity of gray cast iron, the customary metal chosen for the cylinder wall, rather than a more expensive metal of superior heat conductivity;
c) A layer of condensate that forms and is distributed by centrifugal force over the cylinder""s interior;
d) A layer of scale that develops over the cylinder""s internal surface; and
e) A temperature drop required to extract heat from the steam, by condensation.
The difference between the temperature of the steam and that of the cylinder""s external surface represents a waste of energy.
The enormous mass of the cylinder wall and the high inertial load require a large value of installed horsepower capacity and a correspondingly high energy cost to drive the machine.
The above factors that impede energy transfer, plus the thermal inertia of the massive cylinder wall, contribute to a long response time of steam-heated cylinders. The same factors limit the speed and productivity of the machine.
In emergencies such as web breaks, the drying process is upset and the steam valves often fail to respond quickly, filling dryers with varying levels of condensate. The large amount of thermal inertia of the heavy-walled steam-heated cast iron cylinders imposes a long time delay should the dryers require maintenance or clearing.
Recognition of the problems and limitations of steam as the heat source in dryers of paper making machines has prompted proposals of alternative heating media.
It has been proposed that dryer cylinders in paper making machines should be heated internally by electric power; but electricity is inordinately expensive.
It has also been proposed that a dryer cylinder for paper making apparatus should be heated by a flame within the cylinder. Transfer of heat from the gaseous combustion products to the cylinder requires extensive areas of metal exposed to the hot gases and requires efficient removal of the combustion products after their heat has been extracted, so as to provide necessary space that is to receive newly emitted gaseous exhaust. See Hemsath et al., U.S. Pat. No. 4,693,015 issued Sep. 15, 1987, Calhoun U.S. Pat. No. 2,987,305 issued Jun. 6, 1961, and Bourrel et al., U.S. Pat. No. 3,729,180 issued Apr. 24, 1973.
U.S. Pat. No. 4,688,335 issued Aug. 25, 1987 to Krill et al. discloses use of a gas-fired radiant heat generator to heat a cylinder that acts on a web of fibers being pressed against the cylinder by a felt and a nip roller, the web having a large water content. The burner of Krill et al. is in the form of a ceramic fiber matrix shaped as a cylindrical shell. The cylinder""s fiber matrix is to heat the cylindrical shell uniformly about its entire periphery. An air-fuel mixture is supplied to the interior of the shell. The mixture burns as it emerges everywhere from the shell. Unlike Hemsath, above, the energy of combustion in Krill et al. is intended to produce radiant heat. The heated web-engaging cylinder in Krill et al. operates at 600xc2x0 F. to 800xc2x0 F. (315xc2x0 C. to 427xc2x0 C.). That heat is so intense that some of the free water that is present in spaces between the fibers of the web is converted to steam, which blasts other free water through and out of the web. This process is called xe2x80x9cimpulse dryingxe2x80x9d. Even though the supplied air-fuel mixture is adjustable, reduction of the air-fuel supply is limited by the lowest rate needed to sustain combustion. Noting that the type of burner used in Krill at al to produce radiant heat is in the form of a complete cylinder, the heat output would almost certainly be excessive for use in the usual drying section of a paper-making machine, even with its air-fuel supply adjusted downward to a minimum. Moreover, if the temperature of the cylinder were reduced by adjusting the supply of air-fuel mixture for developing a suitable operating temperature at full-speed operation of the apparatus, little if any latitude of downward adjustment would be available for realizing still lower cylinder temperatures as is required during slowed operation of the apparatus.
An earlier form of impulse dryer is acknowledged by Krill et al., referred to in U.S. Pat. No. 4,324,613 issued to Wahren. An external IR burner is used in Wahren to heat an arc of a cylinder""s exterior to a high temperature. The newly formed web with its high moisture content is subjected to intense pressure between nip rollers and the just-heated segment surface of the cylinder to induce impulse drying. The hot segment of the cylinder""s surface is chilled promptly in this process; it is reheated by the external IR burner during on-going rotation. The cylinder""s exterior is a poor heat conductor, to avoid temperature-reducing conduction of heat away from the heated surface, thereby conserving the heat for transfer to the moisture-laden web.
In usual machines for making paper or paperboard, the moist web of fibers is dried by evaporation. The web is constrained against a large surface area of each of many steam-heated cylinders in succession. Despite alternatives that have been proposed for heating the dryer cylinders of apparatus for making paper and paperboard, steam under pressure continues to be the generally accepted heating medium.
A broad object of the invention resides in providing novel heated cylinders. Those cylinders have various applications, but they have attributes of distinctive importance in paper making apparatus. In one aspect of the invention, the heated cylinders (also called xe2x80x9cshellsxe2x80x9d) rotate; a web of material passes partway around each cylinder while maintaining heat-transferring contact with about half of the cylinder""s surface. The cylinder has a horizontal rotary axis. A stationary non-rotatably disposed core in the cylinder includes at least one assembly of gas-fired infrared generators or IR burners which extend along and adjacent to the cylinder but which subtend only an arc or arcs of the cylinder""s interior. The radiant heat of the IR burners is absorbed instantly and directly by that portion of the cylinder""s inner surface which momentarily confronts the IR burners. In operation, the cylinder rotates constantly, but because the IR burners are assembled on the non-rotatably disposed core, the entire inner surface of the cylinder is exposed to the radiant heat. Thus, the cylinder is heated uniformly around its axis by IR burners that confront only part of the cylinder""s interior.
The provision of IR burners extending all along the cylinder but which have only a limited arcuate extent is an aspect of the invention that has profound implications. It makes possible the construction of a cylinder that develops a specified maximum operating temperature, and in like manner it makes possible the construction of a succession of cylinders having either the same specified operating temperature or specified operating temperatures that differ, rising or declining cylinder-to-cylinder, as may be required in treating a web of material. This attribute of the novel cylinders is particularly valuable in paper making machines in which the dryer cylinders comprise the zones of increasing, constant, and falling rates of evaporation. The complements of IR burners in the cylinders are proportioned to develop coordinated operating temperatures of the cylinders at full-speed operation and with maximum supply of air-fuel mixture. In the zones where evaporation occurs at a falling rate, successive cylinders should have progressively higher heat outputs so as to maintain their effectiveness as dryers despite the increasing dryness and poorer heat conductivity from the cylinder into the web.
Each novel cylinder (and multiple cylinders of a machine) has the capacity of being operable over a wide range of temperatures, downward from a maximum, or upward to a maximum, by adjusting its supply rate of air-fuel mixture. This attribute is important in the dryer cylinders of paper making machines, when reducing the operating speed from an established norm and when increasing the speed to the established norm.
IR burners of a novel cylinder are supplied with a combustible air-fuel mixture, ordinarily a stoichiometric mixture of air and fuel. IR burners typically have the distinctive property of converting a large fraction of their energy of combustion into infrared radiation; this is in prominent contrast to burners that rely on transfer of heat by contact of hot combustible gases with surfaces to be heated. Various forms of IR burners are known, including those which have porous ceramic panels, porous sintered metal panels, metal mesh panels, and even ceramic tile plates having a pattern of discrete passages. The form of an IR burner that is best suited to the present purposes is that which is based on the technology of a long series of patents issued to Thomas M. Smith; e.g. U.S. Pat. No. 4,722,681, issued Feb. 2, 1988. See also Derr et al., U.S. Pat. No. 5,464,346, issued recently on Nov. 7, 1995. Such IR burners involve a panel comprised of a porous matrix of ceramic fibers and a binder. The matrix preferably contains material such as silicon carbide particles to enhance the infrared output efficiency of the burners.
The Smith burner is known as an xe2x80x9cinstant-offxe2x80x9d burner. A person""s hand can be placed on the previously radiating face about one second after an emergency shut-down. This rapid response, and low heat-storage reradiating material for the remainder of the stationary heating core, represent a low mass of thermal storage material opposite to the cylinder shell. Upon shutdown, this material cools rapidly; cooling is promoted by the powered removal of the exhaust. Without receiving heat from the heat source, the cylinder shell cools quickly in contrast to steam heat for cylinders. This rapid cool down promotes rapid shut-downs and facilitates any required dryer maintenance.
IR burners are operable over a range of supply of air-fuel mixtures. Throughout the range of supply variations, the combustion occurs at or just inside the exit face of the gas-permeable panel or emitter, heating the surface of the panel to incandescence. When the rate of supply exceeds the maximum, the combustion lifts away from the exit surface of the panel; when the supply drops below a minimum the combustion tends to recede toward the supply face of the gas-permeable panel and combustion ceases. There is a possibility of the burner backfiring; i.e., ignition of the air-fuel combustible supply may occur behind the burner""s panel. The matrix components in the Smith patents are chosen to inhibit backfiring.
Characteristically, the heat output of an IR burner of any particular construction is dependent directly on its area. Increasing the heat output of any given IR burner is achieved by increasing the supply rate of its combustible mixture up to a maximum rate. IR burners are usually operable to produce adjustable rates of heat output. This trait is useful for turning down the temperature of a cylinder and its IR burners correspondingly, for example, when the paper making apparatus is being slowed down.
As will be seen, there are conditions when the air-fuel supply to an IR burner is adjusted somewhat for changing its heat output while the apparatus is in full speed operation. As noted below, part of the turn-down adjustment capability of IR burners of a cylinder is used to advantage for cross-machine profile control. However, it is desirable to reserve most of the turn-down adjustment capability of the cylinder""s IR burners for use when the speed of the apparatus is to be reduced. Accordingly, the designation of the area of the cylinder""s complement of IR burners should be related to its maximum or near-maximum rate of air-fuel supply. This, in turn, is accomplished by designating the arcuate extent of its IR burners of any particular design and efficiency. The terms xe2x80x9ccomplement of IR burnersxe2x80x9d and xe2x80x9cIR burner complementxe2x80x9d means all of the IR burners with which a cylinder is equipped. The term xe2x80x9carcuatexe2x80x9d signifies around the cylinder; xe2x80x9cextentxe2x80x9d signifies a linear dimension, not a number of degrees, so that xe2x80x9cextentxe2x80x9d refers to the width of the IR burners, or to their combined widths if multiple rows of IR burners are used.
IR burners can be made in the form of multiple sections. Each burner may have its own air-fuel supply regulator. However, even though multiple-section burners are used to advantage in the illustrative embodiment of the invention below, it is also feasible to utilize IR burners that are other-than-sectional. In concept, one or more very long IR burners extending along the cylinder may be used, as appropriate, instead of a row of many sectional IR burners.
Another object of this invention resides in utilizing IR burners made in sections to regulate the temperature of annular bands of a drying cylinder selectively, to match or be different from bands at other parts of the cylinder, for developing a desired xe2x80x9ctemperature profilexe2x80x9d across the width of the web being treated. The ends of dryer cylinders that are heated by steam are hotter than the cylinder shell generally. This condition causes the margins of the web to develop xe2x80x9cedge curlxe2x80x9d. Pursuant to one aspect of the invention, edge curl can be controlled by suitably adjusting the air-fuel supply to IR burner sections at the ends of a cylinder. In particular, the temperature of the ends of a cylinder heated by IR burners may have a tendency of tapering down, due to lessened burner-to-cylinder heat transfer or due to greater heat losses at the cylinder""s ends. Different IR burner sections may be chosen or designed in advance to compensate for anticipated temperature deviations, especially declines in temperature at the cylinder ends. This compensation may also be achieved during operation by limited adjustment of the air-fuel mixture supply to the sectional IR burners at the ends of the cylinder or elsewhere as needed. However, as already noted, it is desirable to reserve most of the range of adjustment of the IR burners"" air-fuel supply for use when the speed of the apparatus is being changed.
Equipping drying cylinders with sectional IR burners having separate air-fuel supply regulators affords an excellent means for developing desired profiles of heat output across the width of the web. The apparatus may include a scanning sensor, or multiple stationary sensors may be used for cooperation with respective incremental widths of the web identified with the burner modules inside the cylinder. The series of sensors or the scanning sensor is located downstream of the cylinder having the sensor-controlled burner; it responds to the moisture content of a related incremental width of the web. The sensor or sensors regulate the supply of the air-fuel mixture to individual modules for maintaining a specified moisture content at that portion of the width of the web.
A further object of the invention resides in providing an exhaust duct whose configuration is aimed at avoiding the build-up of hot exhaust gas such as might distort the cross-machine temperature profile of the cylinder. This is of particular concern in paper-making apparatus having cylinders that are very long. Recognizing that IR burners radiate a prominent portion of the heat resulting from combustion, nevertheless the exhaust gas of IR burners is significantly hot. In a horizontal cylinder heated by IR burners extending end-to-end within the cylinder and having a limited arcuate extent, an arcuate space or gap remains in the cylinder which is not occupied by IR burners. In achieving the above object of the invention, an exhaust duct extending end-to-end is located in that arcuate space, above the IR burners. The exhaust gas from the IR burners is strongly impelled upward by its buoyancy. The configuration of the exhaust duct is devised to counteract any tendency of the exhaust gas to develop higher temperatures at some regions along the cylinder than others.
A still further object of the invention resides in providing a cylinder heated by a longitudinally extending IR burner or complement of burners of limited arcuate extent, with means to conserve heat initially absorbed by portions of the cylinder while opposite the IR burners. After a portion of the cylinder that has just been heated leaves the IR burners, the newly heated area of the cylinder radiates heat towards the cylinder""s interior. Pursuant to the just-mentioned object of the invention, heat-absorbing shields are placed all around the cylinder""s interior in regions not occupied by the IR burners or the exhaust duct. These shields become hot and, as such, reradiate heat outward, toward the cylinder, where the reradiated heat is again absorbed by the cylinder.
The heat shields have a further function in the novel cylinder heated by the IR burners. There is a radial clearance space between the rotating cylinder wall and the stationary shields. That space constitutes a passage for the hot exhaust gases emitted by the IR burners; the shields direct the buoyant exhaust to the exhaust manifold. The buoyancy of the hot exhaust gas is strong at all points along the cylinder, thus providing an effective means for removing exhaust gas from the burners all along the length of the cylinder.
The novel cylinders, with their IR burners, have many prominent advantages over cylinders heated by steam, as is customary in the drying section of paper-making machines. Unlike cylinders heated by steam under pressure, where the maximum temperature is limited in practice by the safe pressure-resisting thickness of the cylinder wall, the temperature attainable by the novel cylinder is in no sense limited by the wall thickness of the cylinder. The wall of the novel cylinders may be comparatively thin and lightweight, consistent only with its mechanical requirements; and it may be made of a metal chosen for superior thermal conductivity. The IR burners can be adjusted rapidly to change the cylinder""s operating temperature, and the cylinder wall does not appreciably retard the transfer of heat from the burners to the external surface. The comparatively thin and lightweight cylinders save installed horsepower and driving energy consumption. The IR burners that extend end-to-end along the novel cylinder may comprise sectional burners, whose air-fuel mixture may be regulated selectively and variably to provide and maintain the desired temperature profile across the width of the web being heated, a result not readily attainable with steam-heated cylinders. The novel cylinders are unencumbered by all the problems and consequences of condensate which characterize steam-heated cylinders. The cost and maintenance of high-pressure steam valves are eliminated. The novel cylinders enable the reduction of the required large number of cylinders heretofore heated by steam under pressure, or an increase in the speed and productivity of a paper making machine, or both a reduction in the number of cylinders and an increase in speed. A large number of steam-heated cylinders in the more common type of paper making machine can be replaced by a smaller number of novel cylinders heated to higher temperatures. Alternatively, by using cylinders proportioned for higher-temperature operation than steam-heated cylinders, a paper making machine having many cylinders could be operated at much higher speed, for greatly increased output.
The phenomenon of xe2x80x9cpickingxe2x80x9d is mentioned above. A web emerging from the press section of a paper making machine is commonly cold. If that cold web were to engage a hot dryer cylinder, prominent picking would develop; fibers picked out of the web would stick to the hot cylinder surface. The novel dryer cylinders can readily be constructed to operate at relatively low and incrementally increasing temperatures selected to restrict the temperature differential between the incoming web and each cylinder engaged by the progressively warmer web, thereby to suppress picking. The number of novel IR burner-heated cylinders proportioned to operate at desired low temperatures can be limited by appropriately proportioning their complements of IR burners. A similar difficulty exists at the point where a size-coated web is to enter the afterdryer. The size press cools the web. An external IR burner is provided to apply heat directly to the web, for setting or congealing the size. Nevertheless, the size-coated web would tend to adhere to the first few afterdryer cylinders, an effect that is suppressed by providing cooler cylinders at the beginning of the afterdryer, merely by proportioning the IR burner complement of the novel cylinders appropriately to develop the desired low operating temperatures.
When leaving the main dryer and entering the size or coating station, the web should have a low and uniform cross-machine profile.
Using only steam dryers, sheets are often overdried deliberately to a very low moisture content to insure that the highest moisture across the machine is below the highest target of 4 to 6% sheet moisture by weight. This is performed at the exit end of the zone of falling-rate evaporation. Overdrying is performed in an effort to render inconsequential the non-uniform moisture profile. However, many extra dryer cylinders are needed to remove the last percentages of moisture. In a further effort to achieve coating uniformity, the web is exposed to direct radiation from sectional IR burners distributed across the web; these external IR burners are distributed upstream of the size press and regulated by a cross-machine moisture scanner. This correction of a non-uniform cross-machine moisture profile is achieved more effectively, and without extra space requirement, by a novel cylinder equipped with internal sectional burners and controlled by a cross-machine moisture sensor.
A similar drying condition occurs where the web enters the calender stack. A non-uniform moisture profile across the web can slow production and it tends to cause non-uniform calendering of the web. If no correction of the web is made at the end of the main dryer section, the web leaving the afterdryer has the combined moisture non-uniformities that accumulate in the main section plus non-uniform size or coating moisture.
The conductive heat transfer from a dryer cylinder to a moist web is self-leveling in nature, to a degree; this is because more heat is transferred to colder or wetter areas of the web surface. The novel dryer cylinder can be proportioned to operate at higher temperatures than feasible for steam-heated dryer cylinders. Higher temperature operation promotes cross-machine drying uniformity, due to the self-leveling effect of the conductive heat exchange. When the novel dryer cylinders are proportioned to operate at higher temperature than steam-heated dryer cylinders, not only do they provide faster drying but they also can provide moisture profile correction for especially streaked sheets. Several of these novel dryers in a short series replacing the steam-heated dryer cylinders, both at the end of the falling rate of the main dryer and at the end of the afterdryer section, can offer speed increases and moisture profile corrections well beyond such performance by external IR profilers in current use. Replacing external IR profilers can free valuable production space or it can free space for inserting other process equipment. Of course, if production space is not available, the IR burner of a novel dryer cylinder can have sectional IR burners regulated by a moisture profiling sensor.
The novel cylinders characteristically can readily be constructed to operate (at full speed and with maximum air-fuel supply to the IR burners) over a wide range of temperatures. In meeting the requirements of apparatus for making paper or paperboard, the cylinders can readily be proportioned for operation at the required low temperaturesxe2x80x94such as 100xc2x0 F. (55xc2x0 C.) below the temperature of the constant rate cylinders in the main dryer section. Such low operating temperatures tend to cause large amounts of condensate to form inside steam-heated cylinders. Proportioning the novel cylinders for low temperature operation creates no problem; the arcuate extent of its IR burner is chosen accordingly.
At the opposite extreme, the novel cylinders can readily be proportioned for operation at higher temperatures than safety allows in steam heating practice.
Because the advantage of the novel cylinders over steam-heated cylinders is more marked at some stages of the paper drying apparatus than others, existing paper making and paperboard making apparatus may be improved by substituting novel cylinders in place of steam-heated cylinders that exist in actual paper making apparatus presently in service. And substitutions are distinctly advantageous where moisture profile correction is wanted, because internal modular IR burners can be arranged to heat particular annular bands of a cylinder (under control of moisture profiling sensing devices). The novel cylinders are also highly advantageous as substitutions where higher temperature cylinders are wanted than the available highest temperature steam-heated cylinders, for example at the end of the falling rate zone of the main dryer section and at the end of an afterdryer.
The novel cylinders are also distinctively useful when an external IR burner is used opposite to a novel cylinder for heating both surfaces of a web, also heating the interior of the web without increasing the required space occupied by the apparatus. Radiant energy from the external IR burner penetrates into and through the web. For example, it is known that thick and multi-ply paperboard webs can easily delaminate if heated too quickly, disturbing the newly-formed internal fiber bonds. The same web can easily withstand two-sided heating provided that sufficient moisture has been evaporated and the bonds are set by preceding treatments.
This combination conduction/infrared heat transfer can assist at the wet end of the dryer section, where picking is primarily caused in a cold, moisture-laden sheet being shocked on contact with a hot cylinder surface. Surface fibers loosen and adhere to the heated surface. Picking is reduced as the entering sheet temperature is increased before contacting the dryers, and subsequently the initial dryer temperatures can be increased. Lack of available machine space between the last wet press and dryer section may limit sheet preheating.
This high heat transfer, by conductive exchange from a cylinder to the web and by direct infrared web heating from an external IR burner can be used at the dry end, where thicker paperboard grades inhibit heat transfer more than than lighter grades of paper, and cause the falling rate drying period to be much longer. Most of these thick sheets are not processed with papermaker""s felts in the last dryer section, allowing for an unobstructed exposure of the web which is required for this configuration. The external infrared heating rate can be easily balanced with the inner conductive heating rate to correct for undesirable warping and stresses. The paperboard sheet is not likely to delaminate in this final drying location.
It has been noted above that incremental portions or segments of the width of a web can be subjected to different heating and drying conditions, as by regulating the air-fuel supply to a series of IR burner modules extending across the web. A further object of the invention resides in providing a dryer with a cross-machine sucession of IR burner modules and means to control the air-fuel supply to such modules so as to develop either a uniform cross-machine profile or some other profile that may be desired.
This heating of the web by cylinder-to-web conduction and by direct radiation of heat to the web can also be used in the afterdryer section, after the size press or coating station, where the coating applied to the sheet is sticky and needs to be set quickly. The controllable heat transfer applied simultaneously to both sides of the sheet can speed coating coalescence and minimize unwanted absorption into the sheet. The individual face side and back side heat intensity may be somewhat reduced, but the double-sided heat treatment will be fast.
The following detailed description and accompanying drawings represent various aspects of the invention. While the detailed description relates to paper making machines, some of the novel aspects are applicable to machines for treating webs of other materials. Additionally, it is apparent that some aspects of the invention may be used without others; substitutions and modifications will be readily apparent to those skilled in the art. Consequently, the invention should be construed broadly, in accordance with its true spirit and scope.